The present invention relates generally to gas moisture measurement, and more particularly, to particulate mass measurement instruments operable to measure the moisture content of effluent gas in real-time for real-time adjustment of isokinetic sampling during measurement of the mass of particulate matter flowing in a stack or other exhaust conduit.
Particulate mass measurement of effluent gas flowing in a stack or other exhaust conduit of such stationary sources as coal burning facilities, garbage incinerators, hazardous waste type incinerators, concrete plants, paper/pulp processing plants and the like, is important because of the relationship between particulate matter and adverse health effects. Various regulatory agencies around the world require the continuous mass measurement of particulate matter from stacks.
Obtaining a representative sample of the effluent gas flowing in the stack for particulate mass measurement generally requires that the sample be obtained isokinetically. An isokinetic sample is obtained by maintaining the kinetic energy of the sample as it enters a sampling nozzle of a particulate mass measurement instrument. Kinetic energy is a function of the mass and velocity of the sample. Since the mass of the sample is generally constant, the kinetic energy of the sample may be maintained by matching the velocity of the effluent gas flowing in the stack with the velocity of the sample of the effluent gas flowing through the nozzle. That is to say, an isokinetic sample is one that is drawn with a velocity equal to the velocity of the effluent gas in the stack.
The velocity, V, of the effluent gas flowing in the stack is typically determined using a pitot tube disposed in the effluent gas flowing in the stack according to the following relationship:
xe2x80x83V=Cxc3x97(PitotDP)0.5xc3x97(T/(Pxc3x97Ms))0.5xe2x80x83xe2x80x83(1)
where:
C is the pitot tube calibration constant;
PitotDP is the measured pressure drop across the pitot tube;
T is the absolute effluent gas temperature;
P is the absolute pressure of the effluent gas; and
Ms is the molecular weight of the effluent gas including water vapor.
Typically, the temperature, T, is determined using a temperature sensor or thermocouple, the pressure drop across the pitot tube, PitotDP, is determined using pressure transducers, and the absolute pressure, P, is determined using a pressure transducer.
While the above-noted determinations of temperature and pressures may be made in real-time, the molecular weight of the effluent gas including water vapor, Ms, is user supplied and estimated which results in an approximation of the velocity, V, of the effluent gas flowing in the stack. In particular, the molecular weight of the effluent gas, Ms, is based on the dry molecular weight of the gas (e.g., obtained using carbon dioxide (CO2) and oxygen (O2) sensors, with the remainder of the gas composition assumed to be nitrogen (N2) and guessing at the moisture content, i.e., proportion of water vapor, by volume, in the effluent gas flowing in the stack.
During the sampling, the moisture content of the water vapor is determined, for example, according to U.S. Environmental Protection Agency (EPA) Method 4. In EPA Method 4, a known volume of effluent gas is removed from the effluent gas stream via condensation in a series of chilled impingers and absorption in silica gel. The mass of water collected is then measured and related back to the known volume of the effluent gas to determine the moisture content of the gas stream. Drawbacks with such a method include the moisture content being an average over time and the average moisture content being determined only following the completion of the sampling. The goal is to maintain agreement between the estimated velocity of the effluent gas flowing in the stack and the velocity of the effluent gas (i.e., using the measured average moisture content) within +/xe2x88x9210 percent which is considered satisfactory for achieving isokinetic conditions.
There is a need for an apparatus and method for measurement of the moisture content in an effluent gas in real-time for real-time adjustment of the isokinetic sampling of the effluent gas during particulate mass measurement. More generally, such moisture measurements are needed for determining mass flow and total volumetric flow rate of an emission source in order to calculate the mass emission rate of regulated pollutants. Continuous or real-time moisture measurements are desired because many pollutants are measured on a continuous basis by continuous emission monitors (CEMs).
The present invention provides, in a first aspect, a particulate mass measurement apparatus for measuring mass of particulate of effluent gas flowing in a stack. The apparatus includes a mass measurement assembly for measuring the mass of particulate of the effluent gas flowing in the stack, means for measuring in real-time moisture content of the effluent gas flowing in the stack, and a controller for controlling isokinetic sampling of the effluent gas measurable by the mass measurement assembly based on real-time moisture content of the effluent gas flowing in the stack.
In another aspect, a method for measuring mass of particulate of effluent gas flowing in a stack includes determining a moisture content measurement of the effluent gas in real-time, obtaining an isokinetic sample of a portion of the effluent gas based on the real-time moisture content measurement of the effluent gas, and obtaining a mass of particulate measurement of the isokinetic sample.
In a third aspect, a moisture measurement monitor, includes a first flow sensor for determining a flow rate of a portion of gas having water, a dryer for removing the water from the portion of gas, a second flow sensor for determining a flow rate of the portion of gas without the water, and a controller operably connected to the first flow sensor and to the second flow sensor for determining the moisture content of the gas.
In a fourth aspect, a method for measuring moisture of gas includes determining a first flow rate of a portion of the flow of the gas, removing water from the portion of the flow of gas, and determining a second flow rate of the portion of the flow of the gas without the water.